1. Field of the Invention
The present invention relates generally to a solar battery module including a plurality of solar battery cells which are electrically series-connected and, more particularly, to improvements of a solar battery module including a bypass diode allowing output currents of solar battery cells to be bypassed.
2. Description of the Background Art
With reference to FIGS. 19A, 19B, and 19C, one example of a conventional solar battery module is illustrated. FIGS. 19A, 19B, and 19C show, respectively, a top plan view, a longitudinal cross-sectional view and an equivalent circuit diagram of the solar battery module. In general, a solar battery module 1A includes a plurality of solar battery cells 2 which are electrically series-connected by an interconnector 3 so as to obtain a desired output voltage. In the solar battery module 1A having such structure that the plurality of solar battery cells 2 are series-connected, however, even in a case o where one solar battery cell 2 in the module is shaded, whereas other cells 2 are unshaded, the solar battery module 1A is greatly affected by the single shaded cell, resulting in a substantial decrease in the output of the entire module 1A.
FIGS. 20A and 20B show output voltage-current (V-I) characteristics of one solar battery cell 2. In the graph of each of FIGS. 20A and 20B, the horizontal axis represents voltage V, and the vertical axis represents current I. FIG. 20A shows V-I characteristics provided when one entire solar battery cell 2 is irradiated with light, and FIG. 20B shows V-I characteristics provided when about a half of one solar battery cell 2 is shaded. As can be understood from a comparison between FIGS. 20A and 20B, the output current I of the half-shaded solar battery cell 2 decreases to approximately one-half that of the cell 2 which is entirely irradiated with light.
FIGS. 21A and 21B are similar to FIGS. 20A and 20B and show V-I characteristics of a complete solar battery module 1A. FIG. 21A shows V-I characteristics provided when all of the solar battery cells 2 in the module 1A are irradiated with light, and FIG. 21B shows V-I characteristics provided when approximately one-half of one solar battery cell 2 in the module 1A is shaded. As will be apparent from a comparison between FIGS. 21A and 21B, even in the case where approximately one-half of a single solar battery cell 2 in the module 1A is shaded, the solar battery module 1A is affected such that the output current I of the entire module 1A decreases by approximately one-half.
With reference to FIGS. 22A and 22B, one example of a solar battery system in which a plurality of solar battery modules 1A are further series-connected is schematically shown. FIGS. 22A and 22B show, respectively, a block diagram and an equivalent circuit diagram of that solar battery system. In this solar battery system also, even if only one of the solar battery cells in the system is shaded, the same output decrease as shown in FIG. 21B occurs.
Moreover, in the above-described solar battery module and solar battery system, a large reverse bias voltage is applied to a shaded solar battery cell, so that the shaded cell is destroyed, depending on the external circuit connected with the solar battery module or the solar battery system. In an extreme case, solar battery cells are overheated by the reverse bias voltage, causing a fire in the case where a solar battery module is installed on a roof.
In order to eliminate the problems of the output decrease in the solar battery module or the breakdown of the cell, a solar battery module incorporating a bypass diode is known in the prior art.
FIGS. 23A, 23B, and 23C are similar to FIGS. 19A, 19B, and 19C and illustrate an example of a solar battery module incorporating a bypass diode according to the prior art. FIGS. 23A, 23B, and 23C show, respectively, a top plan view, a longitudinal cross-sectional view, and an equivalent circuit diagram of a solar battery module 1B. FIG. 23D is a perspective view showing one bypass diode. In this solar battery module 1B, bypass diodes 4 are connected in parallel in reverse polarity with respect to respective solar battery cells 2 included in the module 1B.
FIGS. 24A, 24B, 25A, and 25B are similar to FIGS. 20A, 20B, 21A, and 21B, respectively, and show output V-I characteristics of the solar battery module 1B of FIG. 23A. FIGS. 24A and 24B show output V-I characteristics of a single solar battery cell 2 incorporating a bypass diode 4. As can be understood from a comparison with FIGS. 20A and 20B, when there is one solar battery cell 2, the bypass diode 4 does not reduce the decrease in the output of the shaded solar battery cell 2. When a reverse bias voltage externally acts on the solar battery cell 2, however, the bypass diode 4 can act to release the reverse bias voltage and protect the solar battery cell 2 from destruction due to the reverse bias voltage.
FIGS. 25A shows V-I characteristics provided when all of the solar battery cells 2 in the solar battery module 1B of FIG. 23A are irradiated with light, and FIG. 25B shows V-I characteristics provided when approximately one-half of one solar battery cell 2 in the module 1B is shaded.
As can be understood from a comparison between FIGS. 25B and 21B, the bypass diode 4 can act to minimize a decrease in the output of the entire module 1B when a portion of the solar battery cells 2 in the solar battery module 1B are shaded.
With reference to FIGS. 26A, 26B, and 26C, an example of a solar battery module incorporating one bypass diode is illustrated. FIGS. 26A, 26B, and 26C show, respectively, a top plan view, a longitudinal cross-sectional view, and an equivalent circuit diagram of a solar battery module 1C. The solar battery module 1C including a desired number of series-connected solar battery cells 2 includes one bypass diode 4 connected in parallel in a reverse polarity to the solar battery module 1C. When a portion of the solar battery cells 2 in the solar battery module 1C is shaded, this bypass diode 4 does not reduce the decrease in the output of the entire module 1C; however, when a reverse bias voltage acts on the module 1C, the bypass diode 4 can act to protect the entire module 1C from breakdown due to the applied reverse bias voltage.
With reference to FIGS. 27A and 27B, a solar battery system including one bypass diode for each solar battery module is schematically illustrated. FIGS. 27A and 27B show, respectively, a block diagram and an equivalent circuit diagram of that solar battery system. This solar battery system includes a plurality of series-connected solar battery modules 1C, each solar battery module 1C including one bypass diode 4. These bypass diodes 4 can act to reduce the decrease in the output of the entire solar battery system even in a case where a portion of the solar battery cells in the solar battery modules 1C is shaded.
With reference to FIGS. 28A, 28B, and 28C, another example of a conventional solar battery module is illustrated. FIGS. 28A, 28B, and 28C show, respectively, a top view, a longitudinal cross-section view, and an equivalent circuit diagram of a solar battery module 1D. The module 1D of FIGS. 28A, 28B, and 28C is similar to the module 1A of FIGS. 19A, 19B, and 19C; however, with reference to FIGS. 28A, 28B, and 28C, a plurality of relatively small solar battery cells 2 each having a comb-like front electrode 2a are electrically series-connected in such a manner that a portion of each solar battery cell 2 is overlapped with a portion of each adjacent cell 2, in a so-called roof-tile-stacked manner. A back electrode (not shown) is formed on approximately the entire back surface of each of these small solar battery cells 2, and a portion of the front electrode 2a of one solar battery cell 2 is directly attached to the back electrode of its adjacent solar battery cell 2. Solar battery cells 2 on opposite ends of a series of the roof-tile-stacked solar battery cells 2 are connected with interconnectors 3.
In the roof-tile-stacked solar battery module 1D shown in FIGS. 28A, 28B, and 28C also, when a portion of the solar battery cells 2 are shaded, there occurs the same problem of a substantial decrease in output current as described in FIGS. 21A and 21B.
In addition, since the roof-tile-stacked solar battery module 1D has poor flexibility, a crack is liable to be produced in a direction orthogonal to the direction of elongation of that module, as illustrated in FIGS. 29A and 29B. This crack is liable to be produced particularly in a long solar battery module in which a large number of solar battery cells 2 are connected in series. FIGS. 29A and 29B show, respectively, a top view and a longitudinal cross-sectional view of the solar battery module 1D including a cracked solar battery cell 2. When a crack 2K is formed completely across one solar battery cell 2 as shown in FIGS. 29A and 29B, the output of the entire solar battery module 1D decreases down to 0.
The diode 4 of the type shown FIG. 23D is employed in the solar battery module incorporating the prior art bypass diode 4. This diode 4 typically has a cylindrical form of approximately 3-4 mm in diameter and approximately 10 mm in length. On the other hand, the solar battery cell 2 has a thin plate form of, e.g., 100.times.100.times.0.4 mm.sup.3 in size.
Accordingly, when the solar battery module is laminated with and interposed between protection layers, the thin solar battery cell 2 sometimes cracks by being pressed by the thick diode 4. In addition, the overall laminated solar battery module becomes thick due to the thick diode 4, resulting in an increase in the overall weight of the module. Moreover, there is a problem such that if the diode 4 is provided in portions other than the solar battery cell, the size of the entire solar battery module increases by the proportion corresponding to the provision of the diode 4, and the module conversion efficiency (photoelectric conversion efficiency of the module with respect to the size of the module) decreases, resulting in a deterioration in the appearance of the product and a degradation in the value of the commodity. There is another problem that since the diode 4 is thick, a laminating step becomes complicated, resulting in a higher cost of the product.
Moreover, a solar battery module in which solar battery cells having a thin plate form and a relatively small size of, e.g., 30.times.30.times.0.4 mm.sup.3 are roof-tile-stacked, a crack is liable to be produced in the direction orthogonal to the elongated direction of the module as shown in FIGS. 29A and 29B. This results in a problems such that the output of the solar battery module is 0.